Remove duplicates based on column not whole row

Question

I want my code to gather grades-table with students-table with a STUDENT_ID as common factor. With the DISTINCT clause, it removes row duplicates, but I want it to not show the same PERSON_ID more than once, so it will only show each STUDENT_ID once.

It's a huge datafile.

SELECT DISTINCT 
    grades.PERSON_ID, grades.GRADE, 
    students.PERSON_ID, students.ENROLL_PERIOD 
FROM  
    students 
INNER JOIN 
    grades ON students.PERSON_ID = grades.PERSON_ID 
WHERE 
    ENROLL_PERIOD < 132 
ORDER BY 
    students.ENROLL_PERIOD

SAMPLE From before -> STUDENTS:

PERSON_ID   PROGRAM_ID  ENROLL_PERIOD
12401       82          109
12401       4549        112
720921      5921        112
832521      9981        114

SAMPLE From before -> GRADES:

PERSON_ID   GRADE   GRADE_DATE  GRADE_TYPE
2308133     7       16-06-26    p                
2308133     10      18-01-24    p                
2308133     4       16-02-01    p        

SAMPLE from when the 2 tables are combined, it looks like this:

PERSON_ID   GRADE   PERSON_ID   ENROLL_PERIOD
12401       00      12401       109
12401       02      12401       112
720921      00      720921      112
832521      00      832521      114

Here we see, that the "12401" row is there twice, since it's only if the whole row is different It should skip the 2nd "12401" and jump to the next one ( For instance 12491)

PERSON_ID   GRADE   PERSON_ID   ENROLL_PERIOD
12401       00      12401       109
720921      00      720921      112
832521      00      832521      114